Ultrafast differential ion mobility spectrometry at extreme electric fields coupled to mass spectrometry

Microchip-based field asymmetric waveform ion mobility spectrometry (FAIMS) analyzers featuring a grid of 35 mum-wide channels have allowed electric field intensity (E) over 60 kV/cm, or about twice that in previous devices with >0.5 mm gaps. Since the separation speed scales as E4 to E6, these chips filter ions in just approximately 20 micros (or approximately 100-10,000 times faster than "macroscopic" designs), although with reduced resolution. Here we report integration of these chips into electrospray ionization (ESI) mass spectrometry, with ESI coupled to FAIMS via a curtain plate/orifice interface with edgewise ion injection into the gap. Adjusting gas flows in the system permits control of ion residence time in FAIMS, which affects resolving power independently of ion desolvation after the ESI source. The results agree with a priori simulations and scaling rules. Applications illustrated include analyses of amino acids and peptides. Because of limited resolving power, the present FAIMS units are more suitable for distinguishing compound classes than individual species. In particular, peptides separate from many other classes, including PEGs that are commonly encountered in proteomic analyses. In practical analyses with realistic time constraints, the effective separation power of present FAIMS may approach that of "macroscopic" systems.